It is known that a vibratory gyrometer or gyroscope may be manufactured from an axisymmetric or non-axisymmetric resonator with two degrees of freedom. The present invention relates primarily to the first category, namely, to vibratory rate gyroscope manufactured from axisymmetric resonators. Since axisymmetric resonators may be used in vibratory gyroscopes, the present invention also relates to these types of gyroscopes.
The resonator is made to vibrate at its resonant frequency by an amplitude control signal. The orientation of the vibration is controlled by a precession control signal. The demodulation of this signal makes it possible to know, by calculation, the applied precession rate and/or the inertial angle velocity. The excitation of mechanical vibrations of both the principal mode (antinode) and the precession mode (node) is produced by electrical actuators (electromagnetic, piezoelectric or electrostatic actuators), with a gain which is dependent on the ratio of the force generated, which is applied to the resonator, to the amplitude of the mechanical oscillations' excitation signal. The transmission coefficients of the detectors at the antinode and the node are determined by the ratio of the amplitude of the vibrations to the electrical signal being processed, at the output of the detectors.
The precision of calculation of the rate of rotation is a function of the precision of calculation of the scale factor of the manufactured system, and in particular of the precision of determination of the actuators' and detectors' gains of the resonator. The scale factor is the ratio of the rate of rotation of the vibration to the value of the output signal, in the case of a gyrometer. The scale factor of the vibrating gyroscope is the ratio of the rate of the rotation to the value of the precession control signal, in the case of a gyroscope.
The scale factor is a function of temperature and evolves over time. It is common to compensate for temperature variations in the scale factor using a computation unit and temperature measurements, either by tabulation of the scale factor or by a polynomial calculation. These methods may be inadequate and do not take aging into account.
A method for calibrating the scale factor of a vibratory gyroscope is described in US Patent Publication No. 2005259576. The method consists of measuring the output signal of the vibratory gyrometer or gyroscope while it is mounted on a support set and rotated at a constant rate. The scale factor is the ratio of the value of the applied rate of rotation to the value of the output signal. This method may be carried out before the operational use of the sensor using appropriate means. It cannot be carried out while the sensor is in use, unless another sensor is available.
European Patent EP 2 092 271, published on 08.25.2009, proposes two calibration methods, making it possible to improve the precision of the scale factor in the case of an axisymmetric resonator.
The first method adds to the forces controlling the vibration a stiffness control mechanism, making it possible to modulate the frequency of the vibration. Measuring this frequency modulation makes it possible to carry out a confluent analysis. Since the frequency modulation generally does not exceed 1 Hz, and since the average frequency of the resonators that are normally used is between 2 and 20 kHz, the precision required for the frequency measurement for a scale factor with a precision of 0.05% is of the order of 0.25 to 0.025 ppm. Since the resonant frequency of the resonators used is not sufficiently stable versus temperature, this precision of measurement is not fully consistent with an outdoors operational thermal environment.
The second method consists of processing the amplitude control signal using the amplitude detection signal amplified by a constant gain and which is phase-adapted, and in observing the exponentially increasing evolution of this amplitude, then in inverting the sign of the gain to observe the exponentially decreasing evolution of the amplitude, and also in calculating a correction term based on these observations. After this preliminary phase, the amplitude is adjusted to a fixed value. The drawback of this method is that it causes the amplitude, and therefore the scale factor, to vary significantly during calibration. The device is not operational during calibration. After calibration, the benefits afforded by this operation decrease to zero over time.